Among other problems, earthquakes cause gas lines to rupture that in turn release explosive or dangerous gases. For example, natural gas utility lines may rupture to release natural gas. Ruptured gas lines outdoors may release flammable gas but explosive accumulations do not occur. A ruptured gas line indoors however may fill a room with an explosive mixture of gas and air.
Valves that automatically shut off gas lines during earthquakes are known. These usually use inertia as the activating factor. Valves are frequently closed by a pendulum which triggers a switch or otherwise releases a valve closing means. These devices suffer defects among which are that they become rusty or lubricant dries in long periods of disuse and moving mechanisms don't work well, that they do not have graded sensitivity and that they can be triggered by vertical thrusts such as the vibration caused by a train or truck or by being struck accidentally with a hammer.
Some structures are engineered to resist severe earthquakes while others are not. Some structures should continue to have gas supply during moderate earthquakes while others should not. Valves that accomodate to all situations are not generally available.
It is also desirable to shut off gas flow in emergency situations other than earthquakes, for example, when pressures or temperatures of a process run out of control or when utilities are lost. Such valves should respond when situations other than severe lateral thrusts occur and inertia activated valves are not adequate for such situations.
Although the foregoing discussion is related to earthquakes, valves of this nature are also useful when they are activated by explosions or other strong lateral thrusts.